The invention relates to a device as well as a method for exposing a digital image onto light-sensitive material. The apparatus comprises an image generation device that is controlled by a control device in accordance with image data, an optical system for reproducing the image onto the light-sensitive material and a stage or platform for supporting the light-sensitive material
The German Patent No. 42 11 128 C2 describes an image transfer method, where an image original is scanned point by point and line by line, where the generated signals are digitized and where the digital image signals are reproduced on photographic paper via an optical system using a light modulator, which is illuminated by a light source. The light modulator that is controlled in accordance with the image information, may be a light valve tube, a liquid crystal display or an arrangement containing ferro-crystals.
The U.S. Pat. No. 5,796,508 discloses a device for exposing a digital image onto photographic paper which also uses a light modulator. In this device, a DMD (digital micromirror device) is used as the light modulator. This patent points out that it is necessary to monitor the functionality of each individual pixel of the light modulatorxe2x80x94in this case, of each individual mirrorxe2x80x94to obtain a truly excellent image quality on the photographic paper. For this purpose, it is proposed to measure the light reflected by each mirror during the xe2x80x9coffxe2x80x9d phasexe2x80x94that is, when the mirrors reflect the light away from the photographic paper. The measured light is compared to a desired value, which corresponds to the amount of light that the mirrors should reflect under optimal conditions. If a deviation occurs between actual and desired values, the difference is taken into account in the control of the light modulator.
However, such an exposure correction method is only practicable for reflecting-type light modulators such as DMDs, because the control measurement does not interfere with the exposure beam path. This method is not suited for transmitting-type light modulators or for control of the entire system, namely the light modulator including the exposing optics.
Therefore, a principal object of the present invention is to provide a method for exposing a digital image onto light-sensitive material which ensures uniform illumination of the medium to be exposed, and thus, optimum image quality, and which is suitable for any digital exposure apparatus.
This object, as well as other objects which will become apparent from the discussion that follows, are achieved, according to the present invention, by an apparatus and a method for exposing a digital image onto light-sensitive material in which a beam splitter decouples a portion of the light emitted by the image generation device and this light portion is directed onto a light sensor having at least one sensor line.
According to the invention, during exposure of a digital image onto a light-sensitive material, between the image generation device and the light-sensitive material, a portion of the light is de-coupled from the exposure beam path and is directed onto a sensor. The sensor may be either a line sensor or an area sensor; however, it must be capable of obtaining information simultaneously with several pixels. The sensor may be an array of photo diodes or a CCD or any other light intensity measurement device. In the arrangement subject to the invention, the sensor measures, as the actual value, the amount of light that is directed from the image generation device, which is controlled by the control device, to the light-sensitive material. This actual value is dependent upon the control parameters, the current status of the image generation device, and possibly also, of the exposure optics if the de-coupling of the light is carried out after the path through the optics.
To make the actual value independent of the image data, it is also possible to cover the light-sensitive material with a shutter, to illuminate the light modulator and to pass the light on and de-couple it without modulation. In this case, it is advantageous that the actual value is not dependent on the image data; a disadvantage is that the correction cannot be carried out during the exposure but that an extra calibration phase is required.
The actual value is compared to the desired value that is theoretically required to expose the light-sensitive material in order to optimally transfer the image data to the light-sensitive material.
If the actual value deviates from the desired value, the difference is taken into account in the control of the image generation device. By comparing the desired values of the digital image that is to be transferred with the actual arriving light values at the sensor, it can be monitored, whether the image generation device optimally converts the image data into light signals or if deviations from the ideal image transfer values occur. Various error sources can be the reason for such a deviation. Causes for deviations between actual and desired value may be an uneven illumination of the image generation device, but also erroneous or changeable pixels of the image generation device, or an uneven light transmission through the optics in the exposure beam path. These errors are corrected by taking the deviations into account in the control of the image generation device. Thus, the sensor and the image generation device form a control circuit that adjusts the actual transferred amount of light to the one that corresponds to the image value and in this manner ensures an optimal image reproduction.
If each individual pixel of the image generation device is to be correctable, the sensor must be able to obtain the de-coupled light with a resolution that corresponds to at least that of the image generation device. In this manner, fluctuations or errors relating to individual pixels of the image generation device can be optimally correctedxe2x80x94regardless of whether the light modulator is reflecting or transmitting. Irregularities in the illumination of the image generation device caused by the illumination device can be corrected easily in this manner as well. If the resolution of the sensor is lower than that of the illumination device, only large-area fluctuations in the image generation, such as an edge drop of the light source, can be taken into account.
An image generation device that is very cost-effective yet has a high resolution, and for which this invention can be employed in a particularly advantageous manner, is a printer head comprised of several segments. Such a printer head is described in the U.S. Pat. No. 5,767,874, for example. In this patent, several small DMD segments are placed next to each other. To ensure a smooth transition from the image of one segment to that of the next one, it is recommended to allow the images of the segments to overlap slightly, and to modulate the light intensity transmitted by the segments in the overlapping zone with functions that drop the intensity of one image to zero while raising that of the other image to the maximum value, such that the two segments always transmit a consistent value in the overlapping zone.
However, when overlapping the images of the segments, much of the resolution capacity of the DMDs is lost. This can be avoided by placing the images of the segments of an image generation device next to one another without overlapping and by adjusting the transition from one partial image to the next by using the invention described. All methods described in the U.S. Pat. No. 5,801,814 can be used to combine the partial images. Here, an image is split into partial images as well and the partial images are exposed onto photographic paper next to one another and in sequence. In this patent, the overlapping zone of the partial images is corrected by measuring the multiple intensity in the overlapping zone prior to exposing the photographic paper and then adjusting the illumination data in correspondence with this measurement. By having to obtain the data for the correction prior to the actual exposure, much time is lost in this copying process which can be avoided with the present invention, which performs the correction during the exposure. With the design subject to the invention, an overlapping zone is not required, the exposure correction can be performed during the exposure and all known light modulatorsxe2x80x94DMD, DRI (xe2x80x9cDigital Reflective Imagingxe2x80x9d, reflecting LCDs, see the U.S. Pat. No. 5,826,959), GLV (xe2x80x9cGrating Light Valvexe2x80x9d, see International Patent Publication No. WO 98/41893), LCD, PLZT, CRT, LED, etc.xe2x80x94can be used. Particularly well suited are all high-resolution light modulators that can be used in the digital projection technology.
Whenever the digital image is split into various partial images and these partial images must be combined on the light-sensitive material into a total image, the connecting areas of the partial images must be harmonized with one another. This harmonization is necessary regardless of how the partial images are being created or put back together. The partial images can be generated on several light modulators simultaneously and reproduced next to one another, or they can be generated on one light modulator in sequence. In the second case, the partial images are transferred to the light-sensitive material in sequence either by moving the light-sensitive material or the image generation device between the exposures of the partial images or by modifying the image beam path.
According to the invention, at least the light that is emitted by the image generation device and that reproduces the connecting zone and a defined area around it on the light-sensitive material is always split by a beam splitter and a portion of the light is directed to a sensor. To correct the connecting zone of two partial images, the sensor must feature at least two lines such that each line can be assigned to one partial image. The exposure values measured at the two lines are compared and the control is adjusted until the values agree taking into account possible image data.
It is, of course, also possible to split and obtain the light of the entire image. For this, a very large sensor is required.
The beam splitter that de-couples the portion of the light used for the exposure correction can be positioned in front or behind the objective.
If the partial images are combined to a total image by using a beam splitter, as shown in the embodiment of FIG. 2 and described hereinbelow, the second image that is generated at this beam splitter on the side opposite of the objective can be used to correlate the intensity of the exposure in the connecting zones of the partial images.
At the sensor, which may be a CCD, for example, the light obtained at each pixel is compared with the desired value of the light that is to be transferred for this pixel according to the image data. If the values deviate, the control of the respective pixel of the image generation device is adjusted until the actual value and the desired value correspond.
If a sensor with a low resolution is usedxe2x80x94that is, a sensor with a number of pixels that is less than the number of pixels of the image generation device reproduced on the sensorxe2x80x94a 1:1 assignment of the pixels of the image generation device and the sensor is no longer possible. In this case, a specific number of pixels of the image generation device must always be assigned to a pixel of the sensor. The sum of the desired values of the pixels of the exposure device must then be compared to the actual value that is measured at the one pixel of the sensor. Thereafter, the deviations that occur per pixel on the sensor must be compared among adjacent pixels in order to make a statement about an uneven exposure structure.